Whether it's the library or the grocery store or (hmmm, where else do I go?) ...or ...ummm... or the... uhh.. or any other store that is not for groceries, our two and three-quarter year old has been asking, "Is she/he a stranger?" ...which is often followed up by, "Do you talk to them?".
All very good questions since we have been discussing that whole "we don't talk to strangers" thing.
I will say... I much prefer our daughter to blurt this outloud, at the library, all the while pointing, with children and parents sitting all around during story-time verses what she blurted out a couple of times a few weeks ago: "Is that boy black?"